VIEWING GEOMETRY OF AVHRR IMAGE COMPOSITES DERIVED USING MULTIPLE CRITERIA

The U.S. Geological Survey currently generates composites of AVHRR ima gery based on a single objective-maximizing the Normalized Difference Vegetation Index (NDVI)-as a means of reducing cloud contamination. Ou r research supports the findings of others that, in some cases, NDVI i s maximized at the expense of optimal viewing geometry; that is, satel lite zenith angles are often further off-nadir than necessary to ensur e cloud-free viewing. We explore various compositing methods by system atically varying weights on NDVI, satellite zenith angle, and maximum apparent temperature. A test composite of California from September 19 90 appears to be superior to the maximum NDVI and maximum apparent tem perature composites in several respects. First, the satellite zenith a ngle distribution is more closely clustered about nadir, which minimiz es atmospheric path length, spatial distortion, and bidirectional refl ectance effects. Second, neighboring pixels are more frequently select ed with similar viewing geometry and atmospheric conditions.